1. Field of the Invention
The present invention relates to RFID interrogation systems and, more particularly, to monitoring fluid levels using RFID tags.
2. Brief Description of Related Developments
Radio Frequency Identity or Identification (RFID) is a means of storing and retrieving data through electromagnetic transmission to an RF compatible integrated circuit.
Read-only transponders store information that can be electronically “read”. The stored information can be for example, a unique code. In some systems, a signal can be sent to a RFID tag, which charges the tag and allows the information stored in the tag to be returned.
RFID systems have several basic components or technical characteristics that define them. Referring to FIG. 1, generally, these are a reader 2, including an antenna 7 (the device that is used to read and/or write data to RFID tags), a tag 8 (a device that transmits to a reader the data) and the communication between them (RFID uses a defined radio frequency and protocol to transmit and receive data from tags). The reader 2 can be coupled to a computer 4, which might also be connected to a database 6.
RFID tags are generally classified as active tags and passive tags, as defined by their power source.
Active tags contain both a radio frequency transceiver and a battery to power the transceiver. Because there is a transceiver on the tag, active tags have substantially more range (approximately 300 feet or more) than passive or “active/passive tags.” Active tags are also considerably more expensive than passive tags and, as with any battery-powered product, the batteries must be replaced periodically or the product life cycle is less than the battery life.
Passive tags can be either battery or non-battery operated, as determined by the intended application. Passive tags reflect the RF signal transmitted to them from a reader or transceiver and add information by modulating the reflected signal. A passive tag does not use a battery to boost the energy of the reflected signal. A passive tag may use a battery to maintain memory in the tag or power the electronics that enable the tag to modulate the reflected signal. Battery-less (“pure passive” or “beam powered”) tags do not contain an internal power source such as a battery. These purely passive or “reflective” tags rely upon the electromagnetic energy radiated by an interrogator to power the RF integrated circuit that makes up the tag itself.
There is a version of a passive tag that does contain a battery. This type of passive tag with a battery (“active/passive”) has some of the enhanced, and speed attributes of a true active tag, but still communicates in the same method, as do other passive tags. These active/passive tags that do contain an internal power source, usually are much more complex integrated circuits with multiple components.
RF tags can also be distinguished by their memory type. Read/write memory, can be read as well as written into. Its data can be dynamically altered. Read only (typically “chipless”) type of tag memory is factory programmed and cannot be altered after the manufacturing process. Its data is static.
The tags and a reader communicate by wireless signal in a process known as coupling. Two methods of wireless signal distinguish and categorize RFID systems. Close proximity electromagnetic or inductive coupling systems and propagating electromagnetic waves. Coupling is via antenna structures forming an integral feature in both tags and readers.
An inherent weakness of RFID technology is that readings are affected by liquid in the transmission path. If there is liquid between the reader and the tag, reading the tag is not possible and the effects of this increase with the increase of the transmission frequency.